Nonaqueous phase liquid is a term generally used to indicate a water-immiscible organic liquid. The term itself or the acronym NAPL specifically refers to the liquid phase which may in fact comprise one or more different chemicals.
Nonaqueous phase liquids have typically been divided into two general categories, dense and light. These terms refer to the specific gravity, or the density of the nonaqueous phase liquid relative to water. Thus, dense nonaqueous phase liquids have a specific gravity greater than water (DNAPL), and the light nonaqueous phase liquids (LNAPL) have a specific gravity less than water. Examples of common LNAPL pollutants are petroleum products, while organic solvents like perchloroethylene (PCE) or mixtures like creosote or coal tar are common DNAPLs. Among dense nonaqueous liquid phase contaminants, the most frequently encountered are chlorinated solvents.
Subsurface DNAPL contamination is important for several reasons. These liquids are commonly found at hazardous waste sites because of their widespread use in industry. Generally, the solubility of components of DNAPL is relatively low, so that the DNAPL forms a long-lived reservoir of subsurface contamination. Most of the mass of the contaminant resides in the DNAPL phase, which may be confined to a relatively small space, because of the physics of immiscible phase flow. Chemical contamination then originates from a small region and has the ability to contaminate large volumes of water through dissolution of chemicals from the DNAPL. Although solubilities may be relatively low, they are usually high enough to be above drinking water standards. A further problem results from the DNAPLs being more dense than water, so they tend to flow to the bottom of aquifers.
The removal of Dense Non-Aqueous Phase Liquids (DNAPL) from fractured bedrock and saturated soils is one of the most challenging problems facing environmental restoration activities worldwide. Since DNAPLs such trichloroethylene, perchloroethylene, polychlorinated biphenyls and the like are more dense than water, free phase contaminants car sink to great depths through ground water until they encounter an impermeable barrier. In certain locations, the impermeable barrier may be a clay lens or bedrock. However, in other sites the bedrock may be fractured, allowing penetration of the DNAPL throughout the fractures. This complicates remedial activities tremendously since the DNAPL serves as a continual source of dissolved contaminants while making remediation extremely complicated. Conventional pump-and-treat technologies can be applied to the impacted areas for centuries without achieving cleanup objectives because of the relatively low solubilities of the contaminants in ground water. In-situ soil flushing, flooding, solvent extraction, biodegradation, or chemical destruction are precluded by the difficulty in treating DNAPL within the bedrock fractures. The existence of DNAPLs, including chlorinated solvents, is one of the most widespread problem at hazardous waste sites throughout the world. DNAPL contamination has impacted drinking water supplies, spread beyond site boundaries, and is the most complex issue facing environmental cleanup efforts. At sites with fractured bedrock, the cleanup is even more difficult.
Contamination of soils and ground water with chlorinated hydrocarbons is a widespread problem caused by early disposal practices, spills, and leaks of chlorinated solvents, PCBs, pesticides, and petroleum products. Examples of DNAPLs include tetrachloroethane, carbon tetrachloride, trichloroethane, chloroform, dichloroethane, vinyl chloride, methylene chloride, and the like.